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My mac will not boot up. However I went to recovery mode and made a backup of everything on an external drive. My mac only boots up with the external drive now. I have Yosemite installed on my external drive. I tried to backup and restore but it failed. And for the majority of the times my Internal Hard Disk is not mounted. I ran Disk Utility today and got the following message for my HD " Error: Partition Map check failed because no slices were found" How can I fix my Mac and make it start without the external drive.